The object of the present invention is a poly(vinyl alcohol) purification process, as well as a poly(vinyl alcohol) in the form of flakes of a few millimeters in size, or in the form of a powder whose average particle size of greater than 600 .mu.m, and having low methanol and methyl acetate contents.
Poly(vinyl alcohol) is a polymer obtained by reaction of a poly(vinyl ester), generally poly(vinyl acetate), with an alcohol which can be ethanol, or, for an industrial use, methanol. The reaction is carried out with a catalyst which can be a basic catalyst such as sodium hydroxide, or even with an acidic catalyst such as sulphuric or hydrochloric acid. The basic catalyst is preferred for industrial use.
When the alcohol (called PVA in what follows) is prepared in a discontinuous manner, a polymer is obtained as a solid by precipitation in the alcohol used during an alcoholysis, and in the ester released during this same reaction; the precipitation generally takes place in a mixer. A step of neutralisation of the residual catalyst is then carried out, then a washing step of the polymer which is dried later.
When the process is carried out continuously, the product resulting from the reaction is first of all placed in the form of flakes mainly in the form of irregular flakes of 2 to 6 mm and of a thickness of a few tenths of millimeters, by means of a moist grinder, and a step of neutralisation of the residual catalyst is carried out, followed by a washing step. Upon completion of these operations, the polymer is separated as a solid which is dried by any known means. Once the product is dried, a grinding of the dried polymer is generally carried out.
Usually, it is noted that the particle size of the PVA particles obtained by the first process is finer, lower than 200 .mu.m, than with the processes carried out continuously. In fact, by the continuous processes, the particle size of the particles after grinding is greater than 600 .mu.m, even if a priori nothing prevents obtaining a finer particle size via this route. It is to be noted that particles having a larger size are interesting in this sense that they have better properties of use.
It is towards the above-mentioned drying steps that the present invention is directed. In fact, whatever the process used, the PVA resulting from the alcoholysis reaction contains a significant amount of solvents present during the reaction, such as the alcohol used and the ester released. Now, it is necessary to remove these solvents with a view to obtaining contents in the order of a few percent.
However, it is not possible to dry the PVA without precaution since this product is temperature-sensitive. In fact, if it is submitted to a high temperature, greater than or in the order of 100.degree. C., for a significant period of time, it is noted then that some of its characteristics change, such as the water-solubility, the level of hydrolysis, the viscosity and the coloration, which is to be avoided.
A first obvious process for removing these solvents consists simply in heating the PVA at a temperature lower than that from which the polymer starts to degrade. However, such a process is not worthwhile since lowering the temperature leads to significantly increasing the drying time. Furthermore, this type of process does not allow sufficiently removing the alcohol present.
In order to solve this problem of effectiveness, without going to the detriment of the polymer's characteristics, it has been proposed to carry out the purification of the PVA by carrying out a heat treatment of the latter under an atmosphere having a relative humidity of at least 8%, more particularly lower than 15%. Thus, the drying is carried out at temperatures between 90 and 115.degree. C., preferably under vacuum such that the speed of removal of the solvents is further accelerated, which is found to be in the order of a few hours.
However, this process seems more particularly suitable for the drying of PVA particles of small size. In fact, in this case, diffusion into the inside of the particles is facilitated with respect to particles of large size, and from this fact the removal of solvents is rendered easier. Thus, when this type of drying is carried out on a PVA obtained by a continuous process, with a relative humidity such as mentioned above, and at reduced pressure, the resulting PVA has a methanol content and an ester content of 1%.
Another more recently proposed process consists in mixing the PVA to be treated with liquid water in an amount of 10-30% by weight of water with respect to the polymer, such that a swelling of the particles is obtained. The particles are then submitted to a heat treatment in a vertical drier. The poly(vinyl alcohol) is introduced into the head of the drier and passes into a first zone surrounded by a double envelope heated with water. The temperature in this zone is one of the essential points of this process. It is in fact specified that if this temperature was too high, i. e. if the means of heating this zone was vapour and no longer water, the water would have evaporated before being able to act upon the polymer, as a result, the removal of the solvents would not be sufficient. After the passage into this first zone, the PVA is led into two other parts, heated by means of a double envelope comprising vapour, such that the water which is trapped in the polymer particles is evacuated.
This process is efficient but the operation time is a few hours, necessitating the implementation of a vertical drier. Furthermore, this process does not allow treating polymers comprising lower than 10% by weight of solvent with respect to the dry weight of PVA.